1. Field of the Invention
The present invention relates to a technology for correcting color misregistration of an image in an image forming apparatus.
2. Description of the Related Art
In color image forming apparatuses, registration for each color is adjusted to correct skew and curve of an image in an image processing. For correcting skew and curve of an image, the image is generally divided into areas in the main-scanning direction, and then shifted in the sub-scanning direction depending on its skew and curve. However, if an image to which a pseudo halftone process, such as a dither process, has been performed is shifted for correcting skew, dither patterns may be misaligned at a position of shifting of the image, causing a noisy image in which the position of shifting is visible on a printing medium.
It is necessary to correct misregistration for each color in an appropriate manner to form a desired image in the color image forming apparatuses. According to a conventional technology, in general color image forming apparatuses, predetermined toner patterns are formed for each color on a transfer belt, and the toner patterns are detected by an optically-based sensor. Then, values necessary for correcting skew are calculated, such as amounts of misregistration between each of colors, amounts of misregistration in the main- and the sub-scanning direction, amounts of difference in magnification for each color, and amounts of skew or curve. A feedback correction is performed so that each of the above elements becomes identical between each of colors. As a result, misregistration for each color can be reduced. Such a correction process is performed at various timings so that color misregistration can be constantly suppressed within a predetermined range. Examples of the timings include a time of boot of the image forming apparatus, a time when environmental condition, such as a temperature around the image forming apparatus, is changed, and a time when a predetermined number of sheets are printed.
The registration in the main- and the sub-scanning direction can be corrected by adjusting timings of write of image data in the main- and the sub-scanning direction. The magnification in the main scanning direction can be electrically corrected by adjusting a pixel clock. A skew of scanning beam can be corrected mechanically, or by reversely modifying an image to be output in an image processing.
For mechanically correcting the skew of scanning beam, it is necessary to set an adjustment mechanism that displaces a mirror in a write unit. For automatically performing such an adjustment of the mirror, it is necessary to set an actuator, such as a motor, resulting in increasing costs. Furthermore, it is difficult to downsize the write unit. Alternatively, for correcting the skew in the image processing, image data is stored line by line in a line memory, and stored image data is read out line by line, so that skew between each of colors can be corrected. In this case, it is only necessary to set a line memory in an image processing unit in accordance with a correction area. Thus, it is possible to realize correction of skew with lower costs compared to a method of changing mechanical configuration. Furthermore, such a correction can be performed automatically. Therefore, it is widely known that skew can be effectively reduced in the image processing. Moreover, it is known that this method performed in the image processing is also effective to reduce curve due to property of lens provided in the write unit.
Japanese Patent Application Laid-open No. 2001-353906 discloses a conventional technology for correcting skew. In the conventional technology, for correcting registration of an image in each different color, positions of dividing of images for each color are adjusted so that the positions do not come closer to each other. Accordingly, it is possible to prevent degradation of image quality unlike a corrected portion is visible on a printed sheet even after registration is corrected.
However, when skew is corrected in the image processing, a part of image data is stored in a plurality of line memories, stored data is read out line by line, an image is divided in the main scanning direction, and the image is then shifted in the sub-scanning direction, so that skew or curve between each of colors can be reduced. Therefore, dither patterns may be misaligned at a position where the image is shifted. If the dither patterns are misaligned, adjacency relation between pixels in the main-scanning direction is changed (e.g., white pixel is changed to black pixel), so that an area where toners are deposited in the main-scanning direction is also changed. In images to which a pseudo halftone process has been performed, this change occurs frequently and regularly in the sub-scanning direction. Thus, a noisy image is undesirably generated in which a corrected portion in the sub-scanning direction is visible as line on a recorded image (printed sheet).
As described above, if a skew correction (curve correction) is preformed in the image processing, if dither patterns are misaligned at an image shift position, an area where toners are deposited is changed when the image is printed. Such change of the area, where toners are deposited, occurs frequently in the sub scanning direction due to an image processing like a dither process, resulting in causing a noisy image and reducing image quality.